I've used one of these on my mig welder, Goldstar. (On mine) you unscrew the regular nozzle and replace it with the spot welder nozzle. The idea is to press the nozzle up against the panels to be welded, the tabs keeping the nozzle the correct distance from the metal, and weld away (the panels should be clamped together). The wire is still used, of course, in the welding process. Best to practice a bit first using different wire speed and heat settings. I never had a lot of success with this set-up. What worked for me was to drill a 3 or 4mm hole in the outer panel, everyplace I wanted a spot weld, in your case the replacement sill, clamp it or screw it to the inner body metal with small sheet metal screws and zap it through the hole, thereby welding the two bits together. Again, practice first.
As for seam welding the entire bottom length of the sill, I can't imagine lying under the car and trying to weld four feet of thin metal together with a MIG welder. A pro might manage it with little effort but I fail to see the need. Save yourself a lot of grief and spot weld it as the factory did, using weld-through primer. Then shoot cavity wax in each end of the sill after you're done until it runs out between the spot welds. Should do you for as long as you own the car and then some. (See my diy rust treatment on sills.)
Sorry to be so long winded but there was no other way to get it all in.
S
